Xenopus transcription factor IIIA and the 5S nucleosome: development of a useful in vitro system

5S RNA genes in Xenopus are regulated during development via a complex interplay between assembly of repressive chromatin structures and productive transcription complexes. Interestingly, 5S genes have been found to harbor powerful nucleosome positioning elements and therefore have become an important model system for reconstitution of eukaryotic genes into nucleosomes in vitro. Moreover, the structure of the primary factor initiating transcription of 5S DNA, transcription factor IIIA, has been extensively characterized. This has allowed for numerous studies of the effect of nucleosome assembly and histone modifications on the DNA binding activity of a transcription factor in vitro. For example, linker histones bind 5S nucleosomes and repress TFIIIA binding in vitro in a similar manner to that observed in vivo. In addition, TFIIIA binding to nucleosomes assembled with 5S DNA is stimulated by acetylation or removal of the core histone tail domains. Here we review the development of the Xenopus 5S in vitro system and discuss recent results highlighting new aspects of transcription factor - nucleosome interactions.